1. Field of the Invention
The present invention relates to a portable telephone. In particular, the present invention relates to a portable telephone that can decrease influence of electromagnetic wave on a human body and improve antenna efficiency.
2. Description of the Related Art
With widespread use of the portable telephone, improvement of antenna efficiency and various influence of electromagnetic wave in operation are discussed. Especially, in terms of influence on human body, it is necessary to evaluate SAR (Specific Absorption Rate). SAR stands for an amount of electromagnetic energy absorbed per unit time to tissues of unit mass by exposing a human body to electromagnetic wave. As for an apparatus used in the vicinity of a human head, such as the portable telephone, localized SAR is applied for evaluation in Japan. The localized SAR is the average SAR over 10 g of certain portion of localized human body, and there is guidance in Japan that the localized SAR should not exceed 2 W/kg. Thus, it is necessary to design the portable telephone such that the localized SAR of electromagnetic wave from the portable telephone (800 MHz to 900 MHz), used in the vicinity of the human head, is smaller than 2 W/kg.
The article, Wang et al. “Realization of Low SAR in Human Head by Surface Current Suppression on Portable Telephone”, technical report of IEICE (EMCJ98-45), published 1998, pp. 35-40, (hereinafter referred to as “reference 1”) describes that gluing a ferrite sheet with the thickness of 2.5 mm to the head side of the portable telephone housing can reduce the surface current density in the metal body so as to reduce the localized SAR. In addition, the article, Hashimoto et al. “Numerical Discussion on Decrement of Human Body SAR by Means of Shielding Material”, IEICE Vol. J79-B-II No. 8, published 08/1996, pp. 486-491 (reference 2) describes the analysis by use of a phantom model and mentions that it is effective in reducing SAR to shield electromagnetic wave by placing a dielectrics with the thickness of 30 mm between the human head and the portable telephone housing.
In order to improve antenna efficiency while reducing SAR, the article, Hankui et al. “Investigations on Antenna Radiation Efficiency Improvement of Cellular Phones with a Low-Loss Magnetic Plate”, technical report of IEICE (EMCJ2001-31), published 2001, pp. 7-12, (reference 3) describes that a low-magnetic loss magnetic plate with the thickness of 5 mm is effective in improving antenna efficiency, and describes that SAR is reduced by more than 10% with such magnetic plate. Moreover, the article, Watanabe et al., “A Study on Improvement of Antenna Radiation Efficiency for Cellular Phones by a High Permeability Magnetic Sheet”, IEEJ Transactions on Fundamentals and Materials, Abstract of 2002 conference, page 121, (reference 4) describes the analysis, by use of FDTD (Finite difference Time Domain) method, that antenna efficiency is improved by use of the above magnetic plate with the thickness of 0.8 mm.
In U.S. Pat. No. 6,031,731, the antenna efficiency is improved by placing a non-directional chip antenna in a position on the circuit substrate that is separated from and faced to a conductive reflection plate, and by connecting the chip antenna to a ground pattern of the circuit substrate. In addition, incorporating an electromagnetic absorption body, such as a magnetic loss material, in the portable telephone is effective in reducing SAR and preventing influence of unnecessary wave on surroundings (see United States Patent Publication No. 2001/0053763 and JP-A No. 2002-151881).
As described above, various steps are taken for the purpose of improving communication condition of the portable telephone. The above references, however, are not sufficient in consideration of the practice of the portable telephone to require miniaturization while reducing SAR and improving antenna efficiency. For instance, the ferrite and dielectric plates with 2.5 mm to 30 mm in thickness described in the above references 1-3 are not practical in incorporating into the portable telephone while keeping the size and the weight thereof. In the example described in the reference 3, since an electromagnetic absorption body to reduce SAR causes reduction in antenna efficiency, the low-loss magnetics in the portable telephone improves the antenna efficiency by merely 14.7% and reduces the SAR by merely about 10%.
Although the above reference 4 describes that the high permeability magnetic sheet with 0.8 mm in thickness is effective in improving the antenna efficiency, the result is based on a calculation by use of a specific analysis model. This reference does not analyze the influence of other physical value, such as the relative dielectric constant (>1.0) original to the magnetics. Moreover, this reference does not suggest the practical examples.
U.S. Pat. No. 6,031,731 describes the technique to improve the directionality and reduce SAR, but the effect of SAR reduction is merely about 10%. Although U.S. Patent Publication No. 2001/0053763 aims for SAR reduction by interaction between the conductive layer and the wave absorption body with the thickness of 1-4 mm, it is not clear that the physical characteristics of the wave absorption body relates to SAR reduction. Such uncertainty is also applied to the above reference 4. In addition, JP-A 2002/151881 describes the components of the wave absorption body, but is not clear that which physical characteristic of the wave absorption body is effective in absorbing unnecessary wave. Moreover, JP-A 2002/151881 does not mention the specific structure to reduce SAR while keeping the antenna efficiency.